Running web dryers and/or smoke tunnels have exits for web which are formed by a generally horizontal slot. As the web passes through this exit slot, makeup air is drawn into the dryer in a direction opposite to that of the web movement. As a result the web is forced to move up or down based on the fluctuations in the air movement and the volumes of air. This up and down movement of the web is unequal across its width and is referred to as billowing. As the web billows from side-to-side it contacts the downstream roll at one side or the other which causes a tight side of the web and consequent movement of the web to that tight side of the roll. This side-to-side movement is normally attempted to be corrected by means of a web aligner downstream and due to tolerances in the web aligner, some of the movement carries through to the end of the process. The side-to-side web movement may exceed the limits of the web aligners to correct the side-to-side movement.
One example of a web dryer and smoke tunnel with which the present invention finds particular utility is shown in the U.S. Pat. No. 4,462,169 issued July 31, 1984 to Daane. The smoke tunnel designs shown in FIGS. 1 and 2 of that patent are of the type which are intended to be improved upon by the present airfoil. In that patent, as air is drawn back into the smoke tunnel through the sharp edge split orifice 40 in FIG. 1, the air is allowed to expand abruptly within the smoke tunnel and that causes uncontrolled billowing of the web. The arrangement of FIG. 2 of that patent has a different arrangement of the chill rolls when the exit end of the smoke tunnel discharges between the two vertically aligned, adjacent chill rolls.
The airfoils of the present invention can also be used within the dryer housing itself and at the exit end, for instance where a smoke tunnel is not used, but where the web is subsequently engaged downstream by a chill roll, for example. Such housings are shown in U.S. Pat. No. 4,785,986 issued Nov. 22, 1988 to Hella or in the co-pending application Ser. No. 341,816 filed Apr. 24, 1989 by Hella and Perry, or in U.S. application Ser. No. 165,746 filed Mar. 9, 1988 by Hella and Stibbe and which issued as U.S. Pat. No. 4,837,946 on June 13, 1989, all of which have been assigned to an assignee common with this application.